winbuild: MS-DOS batch tidy-ups

- prefer `.bat` extension over `.cmd` for MS-DOS batch, which also
  avoids confusion with OS/400 `.cmd` files.
- cleanup `echo` quotes, drop them consistently.
- delete empty output line from one of the error branches.
- prefer lowercase commands like the rest of MS-DOS batches.
- delete a contraction.
- drop backticks from error message.
- use `nmake.exe` consistently.
- use equal/not-equal operator style consistently.
- inline a single-line `if` branch.
- delete exceptions and rules dealing with Windows `.cmd` extension.

Closes #14084

.gitattributes

@@ -10,7 +10,6 @@ configure.ac eol=lf
 *.sh eol=lf
 *.[ch] whitespace=tab-in-indent
 
-# Batch files (bat,cmd) must be run with CRLF line endings.
+# Batch files must be run with CRLF line endings.
 # Refer to https://github.com/curl/curl/pull/6442
 *.bat text eol=crlf
-*.cmd text eol=crlf